Hemolysis most commonly occurs following bone marrow transplant when there is "minor" ABO blood group incompatibility between donor and recipient. The hemolysis has been attributed to destruction of the patient's incompatible erythrocytes by donor-derived anti-A and/or anti-B antibody produced from "passenger" immunocompetent donor lymphocytes. Extraordinary transfusion requirements of group O erythrocytes in a series of patients receiving unrelated minor ABO-incompatible marrow grafts led us to investigate whether this mechanism could account for the extent of hemolysis observed. In seven consecutive minor ABO-incompatible unrelated-donor bone marrow transplant recipients receiving cyclosporine without posttransplant methotrexate, we observed excessive hemolysis. For cases in this index group, a strongly reactive donor-derived ABO blood group antibody was identified coincident with development of hemolysis. Transfusion requirements in the first three patients (26 U of group O erythrocytes each) greatly exceeded the recipient's volume of incompatible erythrocytes, indicating that lysis of transfused group O erythrocytes was also occurring. Pretransplant erythrocyte exchange transfusion with group O erythrocytes performed in the four subsequent patients decreased the severity of hemolysis, but did not prevent it. Among minor ABO-incompatible marrow graft recipients, an analysis of variance demonstrated effects on transfusion requirements due to donor-recipient relationship being unrelated (P less than .002) and the use of posttransplant methotrexate (P = .0001), and there was interaction between these two factors (P less than .001). Bone marrow transplants from unrelated donors resulted in an exaggerated immune response to ABO blood group antigens, which was associated with hemolysis of transfused group O erythrocytes, as well as the patient's ABO-incompatible erythrocytes. This serious complication may be prevented by posttransplant immunosuppression with methotrexate.